White Streaks In Clear Polycarbonate

I need some suggestions here. I have a new mold that I've been working with and have a lot of problems with start up. I am experiencing a lot of white milky streaks throughout my part during start up. The part is round and about .48 lbs. with a 5 drop manifold. The white streaking will continue throughout the part for the first 3-5 hours of production and it slowly goes away.

I tried a couple special purging compounds that is suppose to aid in the white milkyness experienced in clear polycarbonates at start up, but it doesn't seem to help that much?

The odd part is I can thoroughly purge by barrel before start up and can't see any milkyness in the purge. It sure shows in the part though?

Is there something more to this milkyness that I'm over looking?

I run other cold sprue jobs in clear polycarbonate in other presses and don't have this kind of trouble with the white milkyness?

Any suggestions would be greatly appreciated. I've run out of ideas?

I see the source being gels or unmelted pellets from the injection unit and/or manifold. Secondly I learned the hard way to stop the press with a barrel full of resin, then bank the heats (read ALL) to about 350*F while stopped. On restart wait until the cold screw prevention turns off and immediately start purging the press and manifold. This has been the best method to reduce scrap I've experienced over the years.
Rick n

Purge compound likely is being trapped in HR drops. Can you give HR details?

KOM

brent

The most effective way to purge out polycarbonate is with virgin PC. Purging compounds only do more harm than good with PC. Are you running any other materials before and after molding with the PC as well? One other suggestion may be to run a DOE with your process and see if your barrel temperatures, bp, RPM are too low or too high not allowing your material to melt completely.

Rick,

I read your reply and believe I understand what you are saying, but just want to verify. When you say leave the barrel full are you implying exactly as that states? Or do you mean just run the barrel empty and pull the screw back a little while it's down. Also, what if I am shutting the machine down on a Friday for a restart on Monday? The barrel bank heats of 350F would have to sit unattended all weekend?

Brent,

The HR is a Yudo brand hot runner. I don't have a lot of specifics on it, but will try to find out. The customer opted to go with a China build tool and they said they never heard of a DME hot runner. LOL. Got what the paid for I guess.

Thanks to all for the replies and suggestions.
Dave

Dave-Yes I mean stop the press with a full shot of resin in the barrel and feed throat. Then bank the heats to the 350*F range and let the press sit with the throat cooling on. On startup just raise the temps and as soon as the cold screw prevention goes off, start purging the press. I would not suck the screw back due (without charging for a shot with no suck back) to that air causing degradation of the melt in the barrel.
Rick.

Rick,

Thanks for clarifying. I'll give your recommendations a try and post the results once I know. Might be a week or two before I have any results.

Thanks
Dave